This invention provides a new process for the preparation of insoluble polyvinylpyrrolidone, by means of crosslinking vinylpyrrolidone polymers. Insoluble polyvinylpyrrolidones have heretofore been prepared by various methods. For example, U.S. Pat. No. 2,927,913 and U.S. Pat. No. 2,938,017 teach their preparation by polymerization of vinylpyrrolidone in the presence of various salts and bases, respectively. Insoluble polyvinylpyrrolidones can also be prepared by copolymerization of vinylpyrrolidone with various other monomers such as, for example, cyclic acid amides and oxidizable metals (U.S. Pat. No. 3,759,880, U.S. Pat. No. 3,933,766, German Pat. No. 2,059,484) and other ethylenically unsaturated substances (U.S. Pat. No. 3,941,718). Another method of obtaining insoluble polyvinylpyrrolidones has been to treat the soluble polymer with materials capable of generating free radicals such as hydrazine or hydrogen peroxide (British Pat. No. 1,022,945), an inorganic water soluble persulfate (U.S. Pat. No. 2,658,045) or certain diolefins (Belgian Pat. No. 672,449).
The present invention provides a method for preparing insoluble polyvinylpyrrolidones from the soluble polymers, which method is considerably easier in operation than previous methods and which is capable of producing polymers of varied and controlled cross-linked density.